FAIR VALUE - Asset Measured at Fair Value (Details) - USD ($) $ in Thousands	9 Months Ended
Sep. 30, 2017	Sep. 24, 2016
Fixed Income \| Restricted Investments
Fair Value
Securities transferred to collateral trust account	$ 4,100
Securities purchased and held in the collateral trust account	3,800
Estimate of Fair Value Measurement \| Recurring
Fair Value
Investments at fair value	19,765	$ 11,439
Assets at fair value	19,765	11,439
Estimate of Fair Value Measurement \| Recurring \| Money market funds
Fair Value
Investments at fair value	477	196
Estimate of Fair Value Measurement \| Recurring \| Fixed Income
Fair Value
Investments at fair value	8,204	4,384
Estimate of Fair Value Measurement \| Recurring \| Equity
Fair Value
Investments at fair value	10,194	5,592
Estimate of Fair Value Measurement \| Recurring \| Mutual Funds
Fair Value
Investments at fair value	890	1,267
Estimate of Fair Value Measurement \| Recurring \| Domestic stock funds
Fair Value
Investments at fair value	335	760
Estimate of Fair Value Measurement \| Recurring \| International stock funds
Fair Value
Investments at fair value	87	70
Estimate of Fair Value Measurement \| Recurring \| Target funds
Fair Value
Investments at fair value	260	234
Estimate of Fair Value Measurement \| Recurring \| Bond funds
Fair Value
Investments at fair value	208	203
Estimate of Fair Value Measurement \| Recurring \| Quoted Prices in Active Markets (Level 1)
Fair Value
Investments at fair value	12,447	8,972
Assets at fair value	12,447	8,972
Estimate of Fair Value Measurement \| Recurring \| Quoted Prices in Active Markets (Level 1) \| Money market funds
Fair Value
Investments at fair value	64	64
Estimate of Fair Value Measurement \| Recurring \| Quoted Prices in Active Markets (Level 1) \| Fixed Income
Fair Value
Investments at fair value	1,299	2,049
Estimate of Fair Value Measurement \| Recurring \| Quoted Prices in Active Markets (Level 1) \| Equity
Fair Value
Investments at fair value	10,194	5,592
Estimate of Fair Value Measurement \| Recurring \| Quoted Prices in Active Markets (Level 1) \| Mutual Funds
Fair Value
Investments at fair value	890	1,267
Estimate of Fair Value Measurement \| Recurring \| Quoted Prices in Active Markets (Level 1) \| Domestic stock funds
Fair Value
Investments at fair value	335	760
Estimate of Fair Value Measurement \| Recurring \| Quoted Prices in Active Markets (Level 1) \| International stock funds
Fair Value
Investments at fair value	87	70
Estimate of Fair Value Measurement \| Recurring \| Quoted Prices in Active Markets (Level 1) \| Target funds
Fair Value
Investments at fair value	260	234
Estimate of Fair Value Measurement \| Recurring \| Quoted Prices in Active Markets (Level 1) \| Bond funds
Fair Value
Investments at fair value	208	203
Estimate of Fair Value Measurement \| Recurring \| Prices with Other Observable Inputs (Level 2)
Fair Value
Investments at fair value	7,318	2,467
Assets at fair value	7,318	2,467
Estimate of Fair Value Measurement \| Recurring \| Prices with Other Observable Inputs (Level 2) \| Money market funds
Fair Value
Investments at fair value	413	132
Estimate of Fair Value Measurement \| Recurring \| Prices with Other Observable Inputs (Level 2) \| Fixed Income
Fair Value
Investments at fair value	$ 6,905	$ 2,335

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-Publisher FASB
-Name Accounting Standards Codification
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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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